Current and future electronic systems, such as computer systems and the like, are being required to perform more functions at ever increasing speeds. As a result such electronic or computer systems are being required to operate at increasingly higher voltages. Memory systems are also becoming larger and are being required to operate faster which can also call for higher voltages and power. Accordingly, voltage and current extremes can sometimes occur in these systems causing damage to internal components or adversely affecting performance of some components.
Additionally, in computer systems and the like there is a Peripheral Component Interconnect (PCI) specification or standard for the connection of different peripheral or auxiliary circuit boards or cards to a personal computer (PC) motherboard, processor or the like. Such cards may include graphics cards, disk drive cards, sound cards and the like that can be plugged into PCI slots associated with the PC motherboard. These peripheral cards are typically designed for plug and play applications. A problem with such circuit cards or boards is that they typically are not terminated or grounded and a signal sent to the circuit card can be at least partially reflected back to the source. For compliance with the PCI specification, circuits or systems must be able to handle a voltage doubled signal. For example if a five volt signal is sent, a ten volt signal may be reflected back. A standing wave may effectively be created on the communications bus and the signal that is reflected can become superimposed on the original signal effectively doubling the voltage on the bus.
Voltage protection devices or circuits used to protect against over-voltage, such as reflected voltages in a computer system, are typically connected externally to the semiconductor chip and can occupy considerable area on a semiconductor die or circuit board. Additionally, some voltage protection circuits or devices are not recoverable and may be damaged themselves in the course of protecting another circuit or device from over-voltage and therefore will need to be replaced after a damaging over-voltage pulse or spike. Some protection circuits or devices also may not be able to operate quickly enough to adequately prevent damage to a protected circuit or device under some circumstances. In protecting circuits that both transmit and receive information or data, such as input/output (I/O) circuits, I/O cells of a memory system or the like, it may be desirable to inactivate a protection circuit or device or to insure that the protection circuit or device will not operate when the I/O circuit is transmitting signals or information and to permit the protection circuit or device to be activated or to permit operation whenever the I/O circuit or protected device may be expected to receive signals.
Accordingly, there is a need for a protection circuit or voltage clamp circuit that can be formed as part of the semiconductor chip, that is active and recoverable and that can operate with appropriate speed under the expected operating conditions. There is also a need for a protection circuit or voltage clamp circuit that can be used with circuits or devices that both transmit and receive signals and will not adversely affect operation in a transmit mode or can be disabled in the transmit mode.